An experimental study on the dispersed bubble to slug flow transition in a narrow tube is reported in this work. It shows that the superficial liquid velocity is an increasing function of the superficial gas velocity along the transition boundary. It also shows that the transition always occurs at an average void fraction of 34%. The steady-state dispersion model of turbulent two-phase-mixture is not adequate to explain all these results. An analysis shows that at high flow rates, the transition is governed by the turbulence-dependent rate processes, while at lower flow rates, it is attributed to the formation of Taylor bubbles near injectors. In the former case, the transition is viewed as an effect of simultaneously occurring breakage and coalescence of bubbles which end up with a typical size distribution with some fixed number of critical-or above-critical-size bubbles. This process is formulated on the basis of a bubble population balance and its different constant parameters are evaluated by comparison with the above-mentioned experimental data. The model is used principally to account for the effect of void fraction-dependent turbulence depression, bubble size distribution in the feed and bubble interactions on the process of transition.